The present invention relates to a process for reducing the color of polymethylene poly(phenylisocyanate) compositions and blends thereof. This process comprises mixing a polymethylene poly(phenylisocyanate) with an epoxide, and heating this mixture to a temperature of about 35.degree. C. for at least about 15 minutes, while stirring.
Various blends of isocyanates are known in the field of polyurethane chemistry. A component may be blended with an isocyanate to improve a particular characteristic such as, for example, the tendency to discolor. Organic isocyanates are known to discolor over time. These are, in general, colorless liquids or solids and change from light yellow to brown after as little as a few hours. Different stabilizer packages to prevent discoloration of various isocyanates, including toluene diisocyanates and diphenylmethane diisocyanates and prepolymers thereof, are known.
There are several references in which additives are used to prevent the discoloration of monomeric isocyanates, see for example, U.S. Pat. Nos. 2,885,420,4,677,154 and 4,814,103, as well as in GB 1,465,014. All of these require that the isocyanate be obtained in a colorless or near colorless state, thereby allowing the color inhibitor to be most effective. Once an isocyanate becomes dark, these same additives do not further inhibit or reduce color formation (see, for example, DE 1,793,484). This is especially true of crude MDI (i.e., polymeric MDI), which is inherently dark and extremely difficult to reduce by treatment of the finished isocyanate. In fact, U.S. Pat. No. 5,208,368 discloses that a major disadvantage of the PMDI process is that "this discoloration can only be reduced to an inadequate extent, if at all, even by subsequent phosgenation to prepare the corresponding crude MDI, and that the crude MDI formed cannot be purified by distillation". In other words, once the color forms, it is extremely difficult to reduce it or to remove it.
Two references disclose that the color of PMDI has been reduced by treating the finished PMDI. These are U.S. Pat. Nos. 5,312,971 and 5,583,251. Both of these references describe treating PMDI under strong reducing conditions. In U.S. Pat. No. 5,583,251, PMDI is treated with hydrogen at high temperatures (200.degree. C.) in the presence of a hydrogenation catalyst to obtain color reductions of up to 50% for the absorbance at 430 nm and up to 77% for the absorbance at 520 nm. In U.S. Pat. No. 5,312,971, PMDI treated with a strong chemical reducing agent (i.e., borane: tetrahydrofuran complex) gave mixed results. The 430 nm absorbance decreased by 21% but the 520 nm absorbance increased by 27%.
U.S. Pat. No. 2,885,420 describes stabilizing agents which can be added to organic monomeric isocyanates which have been distilled in quantities of from 0.01% to 0.5% by weight to prevent discoloration of the isocyanates. These stabilizing agents are essentially colorless compounds of the following group: aliphatic, aromatic and cycloaliphatic ethers and thioethers and mixtures of this group of compounds. These compounds are essentially non-reactive with organic isocyanates under ordinary atmospheric conditions and/or react slowly in the absence of a catalyst. This group of compounds stabilizes the organic monomeric isocyanates against discoloration, even in the presence of oxygen, moisture or light.
U.S. Pat. No. 4,677,154 describes a stabilizer package comprising 2,6-di(t-butyl)-p-cresol (BHT) and a second compound may be added to a thermoplastic polyurethane reaction mixture in quantities of from 0.01 to 1% by weight, based on the weight of the isocyanate, to eliminate discoloration in the resultant polyurethane. The second compound of the stabilizer package is selected from various phosphite, diphosphonites, substituted phenols, etc. The reaction mixture also comprises 4,4'-methylene diphenyl diisocyanate, a polyol selected from the group consisting of polyester polyols and polyoxyalkylene polyether polyols, and a short chain extender. This stabilizer package is suitable for use to prevent discoloration of polyurethanes based on various isocyanates including aliphatic, cycloaliphatic, arylaliphatic and aromatic isocyanates, including polymeric polyisocyanates such as polymethylene poly(phenylene polyisocyanate).
Similarly, U.S. Pat. No. 4,814,103 describes the use of a hindered phenol, such as, for example, BHT and aliphatic epoxides to prevent discoloration of prepolymers based on monomeric MDI, especially MDI with a high 2,4'-monomer content. The hindered phenols must be added to the 2,4-rich MDI immediately after distillation, while the epoxide can be added then or later along with the co-reactants for preparing the prepolymer.
A process for improving the storage stability of diphenylmethane diisocyanates is also described in GB 1,465,014. This process comprises storing refined diphenylmethane diisocyanate in intimate admixture with glycidol. Suitable diphenylmethane diisocyanates include the 2,2'-isomer, the 2,4'-isomer and the 4,4'-isomer, and mixtures thereof. It is also possible that trace quantities of related triisocyanates such as, for example, 2,4'-di-(p-isocyanobenzyl)phenyl isocyanate as well as other impurities such as ureas, carbodiimides, uretonimines, etc. are present in minimal amounts. The quantity of glycidol is typically from 0.001 to 0.25% by weight of the isocyanate. It is also possible that glycidol be used in combination with an organic solvent such as, for example, toluene, dimethyl phthalate, acetone, etc. The isocyanate can be dissolved in the solvent, the glycidol added to that solution and the solvent removed, or the glycidol can be added to the diisocyanate as a solution in a small amount of an inert solvent.
All of these patents are directed to stabilization of polyurethane compounds against discoloration. Most polymeric MDI mixtures are highly colored when they are manufactured. In fact, many of these products are residues and thus, the reproducibility and standardization of these polymeric MDI blends is cumbersome.
Fire-retardant polyurethanes and polyurea-urethanes having improved processability and color stability are disclosed by U.S. Pat. No. 3,919,166. These comprise the reaction product of an organic compound having active hydrogen-containing groups that are reactive with isocyanate groups, an organic polyisocyanate, and 2,3-dibromo-2-butenediol-1,4, and, optionally, a long chain a-olefin epoxide. In another embodiment, the long chain a-olefin epoxide can be melt-blended in after the product is formed. These polyurethane compositions are described as having improved stability against deterioration resulting from exposure to heat or to sunlight or ultraviolet light. The addition of the epoxide to the reaction mixture effectively inhibits discoloration and facilitates processing of the polyurethanes.
Epoxides are also disclosed as effective stabilizers against thermolysis and contact discoloration in polyurethane-forming mixtures by U.S. Pat. No. 4,775,558. These polyurethane-forming mixtures comprise an organic polyisocyanate, a high molecular weight isocyanate reactive material, a catalyst and an epoxide as a stabilizer. Suitable epoxides have equivalent weights of between 57 and 10,000, and are used in amounts of 0.01 to 50 parts by weight, based on 100 parts by weight of polyether polyol.
A method of treating toluene diisocyanate distillation residues is disclosed by U.S. Pat. No. 4,904,704. This method comprises treating the residues with at least about 0.5 equivalents, based on hydrolyzable chloride concentration of the residues, of an epoxy compound at a temperature of from about 155 to 220.degree. C. This results in a treated distillation residue having a hydrolyzable chloride level of less than about 800 ppm. These TDI distillation residues are useful for producing rigid foams.
A process for the standardization and stabilization with simultaneous increase of reactivity of organic polyisocyanates is described by U.S. Pat. No. 5,342,881. This process comprises mixing an organic polyisocyanate at 20 to 150.degree. C. with a) 0.01 to 10% by weight, based on the polyisocyanate, of an organic compound corresponding to a specified formula which contains at least one epoxide group, and b) 0.01 to 1% by weight, based on the polyisocyanate, of a silylated acid corresponding to the specified formula therein.
U.S. Pat. No. 5,208,368 discloses the preparation of mixtures of diphenylmethane diisocyanates and polyphenylpolymethylene polyisocyanates of reduced iodine color number. This process for the preparation of crude MDI of reduced iodine color number by reacting the corresponding crude MDA with phosgene in the presence of at least one inert organic solvent at elevated temperature, removing excess phosgene and solvent when the phosgenation is complete, and heating the reaction product which comprises incorporating monohydric or polyhydric polyoxyalkylene alcohols in an effective amount into the reaction mixture when the phosgenation is complete. This addition of monohydric and/or polyhydric polyoxyalkylene alcohols can substantially reduce the iodine color number of crude MDI, for example, to values of less than 60, preferably of from 35 to less than 20.
U.S. Pat. Nos. 3,793,362 and 3,925,437 relate to methods of reducing the acidic impurities in organic polymeric isocyanates. U.S. Pat. No. 3,793,362 describes a process to reduce the acid content and hydrolyzable chlorine content by mixing polymethylene poly(phenyl-isocyanate), PMDI, with from about 0.25 to 1.0 epoxy equivalents of a monomeric epoxide compound, for each equivalent of acid present in the PMDI. Suitable monomeric epoxide compounds contain at least one epoxy group present in a substituent attached to an aliphatic, cycloaliphatic or aromatic hydrocarbon, or have two terminal epoxy groups separated by the residue of a polyoxyalkylene glycol having a molecular weight of from about 100 to 700.
It is disclosed by U.S. Pat. No. 3,925,437 that the acidity of methylene-bridged polyphenyl polyisocyanate mixtures can be reduced by mixing the isocyanate mixture with a lower alkylene oxide compound that contains from 2 to 4 carbon atoms per molecule. In this process, the mixing may occur at ambient temperatures or in the presence of heat.
DE 1,793,484 also discloses that the addition of epoxides to polymeric MDI stabilizes the reactivity of the polymeric MDI by reducing the acidity. Epoxides are added to the polymeric MDI in amounts of from 0.15 to 3% by weight, based on the weight of polymeric MDI. This reference also states that color stabilization of colorless isocyanates can be attained by addition of epoxides. Epichlorohydrin, styrene oxide, bisphenol A diglycidyl ether, and N,N-(diepoxypropyl)aniline are disclosed as suitable epoxides. However, the addition of epoxides to dark colored isocyanates, such as, for example, polymethylene poly(phenylisocyanate (i.e., PMDI), does not reduce the color of these isocyanates.
Mixtures of various groups of epoxides and polymethylene poly(phenylisocyanate) blends is disclosed by U.S. Pat. No. 5,726,240. The addition of these epoxides is disclosed as stabilizing the reactivity of the polyisocyanate blends such that it is no longer dependent on the acidity of 25 the polyisocyanate blend. This reference broadly discloses that suitable mixtures comprise a) 90 to 99.5% by weight of polymethylene poly(phenyl-isocyanate) blends, and b) 0.5 to 10% by weight of epoxides having an epoxide equivalent weight of about 44 to 400.
According to the present invention, it is now possible to reduce the color of polymethylene poly(phenylisocyanate) blends by mixing with an epoxide, and heating this mixture to a minimum temperature of 35.degree. C. for at least about 15 minutes. The advantage of the present invention is a significant reduction in polymethylene poly(phenylisocyanate) color under mild conditions. These conditions are relatively mild in comparison to other methods known and described in the literature and prior patents as set forth above. The above background information illustrates the difficulty of decreasing the color of PMDI once it forms in the final polymeric isocyanate. It also demonstrates the usefulness of the present invention. By simply adding small amounts of epoxides to the final polymeric isocyanate, significant color reductions can be achieved under mild conditions. The processing advantages of the present invention over the techniques disclosed in U.S. Pat. Nos. 5,312,971 and 5,583,351 are obvious: mild reagents, low temperatures, and low pressure (atmospheric). The present process allows for color reductions in treating the final polymeric isocyanate which are similar to those disclosed in U.S. Pat. No. 5,583,251 to be achieved, and better than those in U.S. Pat. No. 5,312,971.